The present invention relates to an apparatus for monitoring optical signal-to-noise ratio of optical signals in wavelength-division-multiplexing (WDM) optical transmission system, and more particularly, to an apparatus for monitoring optical signal-to-noise ratio of each optical signal by measuring noise signal occurring when an optical signal is detected through de-multiplexing optical signals at each channel in wavelength-division-multiplexing (WDM) optical transmission system.
Wavelength-division-multiplexing (WDM) optical transmission system is a system that transmits several transmission lasers with different wavelengths from each other by multiplexing them in an optical fiber.
By using the system, there is an advantage to significantly increase transmission capacities per optical fiber, even though each laser operates with relatively low transmission rate.
It is necessary to use an optical fiber amplifier for amplifying optical signals so as to increase transmission range in these systems. However, due to amplified spontaneous emission (ASE) light, occurring when the optical fiber amplifier amplifies optical signals, optical signal-to-noise ratios of the optical signals are deteriorated and therefore, that causes performance degradation of total system.
That is to say, as optical signal-to-noise ratio of an optical signal is directly related to the performance of system, the performance of wavelength-division-multiplexing (WDM) optical transmission system can be measured by monitoring optical signal-to-noise ratio. Also, more effective maintenance of a system is achieved by comprehending precise performance of the system.
Especially, in the case of all optical transmission networks, which is expanded wavelength-division-multiplexing (WDM) optical transmission system, due to different optical signal-to-noise ratio of an optical signal at each channel, the monitoring of optical signal-to-noise ratio for each optical signal is indispensable.
Conventional method for monitoring optical signal-to-noise ratio of an optical signal was using optical spectrum analyzer with rotating diffraction grating.
Even though these optical spectrum analyzers have advantages of wide measurement range and high accuracy, there is disadvantage of additional installation cost in wavelength-division-multiplexing (WDM) optical transmission system caused by high volume and high cost.
Several methods for monitoring wavelength-division-multiplexing (WDM) optical signal-to-noise ratio of an optical signal, while complementing the disadvantage, have been proposed.
First, there is one technique of xe2x80x9cSignal Monitoring Apparatus for Wavelength-division-multiplexed Optical Communicationxe2x80x9d [U.S. Pat. No. 5,796,479], which was issued for a patent by Dennis Derickson and Roger Lee Jungerman and registered.
This method separates wavelength-division-multiplexing (WDM) optical signals, which are incident upon via an optical fiber by using diffraction grating, at each wavelength and then monitors optical signal-to-noise ratio of the optical signal by using photo diode array.
However, this method has problems of low accuracy in measurement and instability of optical spatial alignment on account of spatial distance between the optical fiber and the diffraction grating.
Next, another technique was described in a paper entitled xe2x80x9cA High-Performance Optical Spectrum Monitor with High-Speed Measuring Time for WDM Optical Networksxe2x80x9d written by K. Otsuka, Y. Sampei, Y. Tachikawa, N. Fukushima, and T. Chikama, in xe2x80x9c97 European Conference on Optical Communication, pp. 147-150, 1997xe2x80x9d. As this method also used a diffraction grating and photo diode array, there were problems of instability in optical spatial alignment and low accuracy in measurement.
Next, another technique was described in a paper entitled xe2x80x9cHigh Resolution Fiber Grating Optical Network Monitorxe2x80x9d written by Chris Koeppen, Jefferson L. Wagner, Thomas A. Strasser, and John KeMarco, in xe2x80x9cNational Fiber Optic Engineers Conference ""98, Sep. 14-17, 1998xe2x80x9d. This method used a blazed bragg grating and photo diode array.
However, this method had problems of not having precise measurement result of optical signal-to-noise ratio of an optical signal unless spatial alignment between the blazed bragg grating and photo diode array is stable.
Besides, there is method for monitoring optical signal-to-noise ratio of an optical signal by using Fabry-Perot Filter.
However, these methods for monitoring optical signal-to-noise ratio of an optical signal are useful only when optical signal-to-noise ratios are nearly same and the characteristic of amplified spontaneous emission (ASE) light, occurring from optical amplifier, is flat. Particularly, in the case of all optical transmission network, which is expanded wavelength-division-multiplexing (WDM) optical transmission system, as each channel is added/dropped by an optical add-drop multiplexer, transmission range at each channel is different from each other.
Therefore, the intensities of amplified spontaneous emission (ASE) lights, which are occurring from optical amplifier, are different from each other and as a result, optical signal-to-noise ratio at each channel is different.
FIG. 1 shows optical spectrums in wavelength-division-multiplexing (WDM) optical transmission network, after passing several optical add-drop multiplexers. Referring to FIG. 1, it is known that the spectrums of optical noises, occurring from optical amplifier, does not have flat characteristics by arrayed waveguide grating within optical add-drop multiplexer and optical signal-to-noise ratio at each channel is different from each other.
Moreover, as the optical signal is distorted by optical filter such as array waveguide grating, core device of optical add-drop multiplexer, these conventional methods have problem of impossibility in the measurement of optical signal-to-noise ratio.
FIG. 2A and FIG. 2B show optical spectrums measured by optical spectrum analyzer with resolution of 0.05 nm after passing an optical signal with 25 dB of optical signal-to-noise ratio through wavelength-division-multiplexing optical add-drop multiplexer.
In the FIG. 2A is capable of monitoring optical signal-to-noise ratio when passing through optical add-drop multiplexer with passband of 1.1 nm. However, FIG. 2B is difficult to monitor optical signal-to-noise ratio when passing through optical add-drop multiplexer with passband of 0.3 nm.
Due to the low resolution, the measurement of optical signal-to-noise ratio is impossible in the case of FIG. 2B, which is impossible to be measured by optical spectrum analyzer with high resolution. Therefore, new method for monitoring optical signal-to-noise ratio, which is also applicable to wavelength-division-multiplexing (WDM) optical transmission network, is indispensable.
An object of the present invention is to provide an apparatus for monitoring optical signal-to-noise ratio in wavelength-division-multiplexing optical transmission system, in which maintenance and management of system can be effectively performed by de-multiplexing the optical signal of each channel in wavelength-division-multiplexing optical transmission system, applying the signal to optical detector, and then monitoring optical signal-to-noise ratio from the quantity of noise occurring when the applied optical signals are detected by the optical detector.
To achieve the object according to the present invention, the present invention applicable to optical add-drop multiplexer, in which wavelength-division-multiplexed optical signals are split at each channel, provides an apparatus for monitoring optical signal-to-noise ratio in wavelength-division multiplexing optical transmission system, comprising: an optical splitting means for splitting optical signals applied from an external; an optical power measuring means for measuring intensities of optical signals out of a portion of said split optical signals; a noise measuring means for measuring intensities of noises occurring when detecting optical signals out of other portions of said split optical signals; and an optical signal-to-noise ratio (OSNR) calculating means for calculating optical signal-to-noise ratio from both said intensities of the optical signals and said intensities of the noises.
Preferably, said optical splitting means is either star coupler or grating device of an optical fiber for providing a portion of optical signals to both said power measuring means and said noise measuring means, after passing most of optical signals out of optical signals applied through an arbitrary optical fiber.
Preferably, said optical power measuring means is an optical power monitor for measuring intensities of optical signals out of optical signals applied through said optical splitting means.
Preferably, said optical power measuring means includes an optical signal detecting means for transforming optical signals, applied through said optical splitting means, into electric signals and an amplifier for measuring intensities of optical signals after amplifying only DC elements out of the electric signals detected from said optical signal detecting means.
Preferably, said noise measuring means includes an optical signal detecting means for transforming optical signals, applied through said optical splitting means, into electric signals, an AC amplifying means for amplifying only noise elements from said optical signal detecting means, and a noise intensity measuring means for measuring noises of the optical signal from said AC amplifying means.
More preferably, said AC amplifying means includes a capacitor cutting off low frequency for passing only AC elements and cutting off DC element of the detected optical signals from said optical signal detecting means, and an amplifier for amplifying AC elements having passed said capacitor.
Preferably, said noise intensity measuring means includes a analog-to-digital converting means for converting electric signals of the noise element being output from said AC amplifying means into digital signals, a fast fourier transforming means for performing fast fourier transformation on the converted digital signal from said analog-to-digital converting means, and a noise intensity calculating means for calculating noise intensity by using said fast fourier transformed value.
Preferably, said noise intensity measuring means includes an electric filtering means for extracting only noise elements of an optical signal detecting means after electrically filtering the electric signal of noise elements being output from said AC amplifying means and a power sensing means for detecting noises"" intensities from the output of said electric filtering means.
Preferably, said optical signal-to-noise ratio (OSNR) calculating means calculates optical signal-to-noise ratio (OSNR) by applying such as intensities of optical signals (Ptotal) being monitored from said optical power measuring means, noises"" intensities (Ntotal) being monitored from said noise measuring means, non-beat noise (Nnonbeat) already monitored, resolution (R), bandwidth of the optical signals (Bo), amplification constant (A) of optical signal detector and AC amplifying means, etc. to following Equation:                                                                                           N                  total                                =                                                      N                    beat                                    +                                      N                    shot                                    +                                      N                    thermal                                    +                                      N                    circuit                                    +                                      N                    signal                                                                                                                          =                                                      N                    beat                                    +                                      N                    nonbeat                                                                                      ⁢                  
                ⁢                              P            total                    =                                                    P                sig                            +                              P                ase                                      =                                          P                sig                            ⁡                              (                                  1                  +                                                            1                      OSNR                                        ⁢                                                                  B                        o                                            R                                                                      )                                                    ⁢                  
                ⁢                                                                              N                  beat                                =                                  A                  ⁡                                      (                                                                  2                        ⁢                                                  P                          sig                                                ⁢                                                  P                          ase                                                ⁢                                                  1                                                      B                            o                                                                                              +                                                                        P                          ase                          2                                                ⁢                                                  1                                                      B                            o                                                                                                                )                                                                                                                          =                                  2                  ⁢                                      A                    R                                    ⁢                                                            P                      sig                      2                                        ⁡                                          (                                                                        1                          OSNR                                                +                                                                              B                            o                                                                                2                            ⁢                                                          R                              ·                                                              OSNR                                2                                                                                                                                                        )                                                                                                                              [        Equation        ]            
Where, A is constant, and Psig, Pase, and Bo are intensity of signal, intensity of amplified spontaneous emission (ASE) light, and bandwidth of optical signal respectively, and OSNR is optical signal-to-noise ratio being monitored with resolution of R.
Preferably, said optical signal-to-noise ratio (OSNR) calculating means calculates optical signal-to-noise ratio (OSNR) by applying such as noise intensities of noises (Ntotal) being monitored from said noise measuring means, non-beat noise (Nnonbeat) already monitored, resolution (R), bandwidth of the optical signals (Bo), amplification constants (C and D) of optical signal detector and AC amplifying means, etc. to following Equation:                                                                                           N                  total                                =                                                      N                    beat                                    +                                      N                    shot                                    +                                      N                    thermal                                    +                                      N                    circuit                                    +                                      N                    signal                                                                                                                          =                                                      N                    beat                                    +                                      N                    nonbeat                                                                                      ⁢                  
                ⁢                              P            total                    =                                                    P                sig                            +                              P                ase                                      =                                          P                sig                            ⁡                              (                                  1                  +                                                            1                      OSNR                                        ⁢                                                                  B                        o                                            R                                                                      )                                                    ⁢                  
                ⁢                                                                                                  N                    beat                                    ⁢                                      xe2x80x83                                    [                  dB                  ]                                =                                  xe2x80x83                                ⁢                                                      10                    ⁢                                          xe2x80x83                                        ⁢                                          log                      ⁡                                              (                                                                              2                            ⁢                            A                                                    R                                                )                                                                              +                                      2                    ⁢                                                                  P                        sig                                            ⁢                                              xe2x80x83                                            [                      dB                      ]                                                        -                                      C                    ·                                          OSNR                      ⁢                                              xe2x80x83                                            [                      dB                      ]                                                                                                                                              =                                  xe2x80x83                                ⁢                                  D                  +                                      2                    ⁢                                                                  P                        sig                                            ⁢                                              xe2x80x83                                            [                      dB                      ]                                                        -                                      C                    ·                                          OSNR                      ⁢                                              xe2x80x83                                            [                      dB                      ]                                                                                                                              [        Equation        ]            
Where, C and D are constants, and Psig, Pase, and Bo are intensity of signal, intensity of amplified spontaneous emission (ASE) light, and bandwidth of optical signals respectively, and OSNR is optical signal-to-noise ratio being monitored with resolution of R.